1. Field of the Invention
The present invention relates to displacement sensors and, more specifically, to a displacement sensor that uses a beam of electromagnetic radiation to measure displacement of a probe.
2. Description of the Prior Art
Atomic Force Microscopes (AFM) are used to measure surface characteristics of materials at the nano scale. AFM's are useful in measuring thin and thick film coatings, ceramics, composites, glasses, synthetic and biological membranes, metals, polymers, and semiconductors, among many other things. By using an AFM one can not only image the surface with near atomic resolution but can also measure the forces of the surface of a sample at the nano-Newton scale.
An AFM typically includes a probe with a probe tip extending therefrom. One type of probe is a cantilever; another is a force sensing integrated readout and active tip (FIRAT) probe. A cantilever includes a beam with a probe tip at a distal end. The beam may be angularly deflected to move the probe tip toward the object being measured. When the probe tip begins to interact with the object, the deflection of the beam can be measured by sensing light from a laser reflected off of the beam. A FIRAT probe includes a membrane that is supported by a frame. A FIRAT probe tip is typically affixed to the center of the membrane. The membrane may be displaced vertically by one of several methods. In one such method, the membrane has a first conductive surface that is spaced apart from a second conductive surface. When a similar charge is applied to both the first conductive surface and the second conductive surface, the two surfaces repel each other, thereby forcing the membrane (and the probe tip) away from the second conductive surface. The vertical displacement of the probe tip is controlled in this way. The vertical displacement of the membrane is detected by reflecting light from a laser off of the membrane and passing the reflected light through a diffraction grating and then measuring the intensity of one or more modes of the diffracted light using a photodetector.
A diffraction grating is a reflecting or transparent substrate whose surface contains fine parallel grooves or rulings that are equally spaced. When light is incident on a diffraction grating, diffractive and mutual interference effects occur, and light is reflected or transmitted in discrete directions, called orders. A diffraction grating includes a transparent surface with a plurality of parallel lines scored in the surface or printed on the surface and spaced apart at a distance so that they cause a beam of light at a predetermined wavelength to diffract. A one-dimensional diffraction grating includes one set of parallel lines, whereas a two-dimensional diffraction grating includes two sets of parallel lines transverse to each other. Certain natural substances and synthetic substances are diffraction gratings due to their ordering of unit cells in their molecular structures. For example, certain minerals act as diffraction gratings. Thus, certain crystals are diffraction gratings and, if they have greater than a nominal thickness, they are three-dimensional (or even multi-dimensional) diffraction gratings. A diffraction grating with more than one dimension will generate more than one diffracted beam, in which each beam can correspond to a different order. Also, a diffracted beam includes a “bright field” central portion and a “dark field” fringe portion. The central portion can provide information about one aspect of the surface from which the diffracted beam originates, whereas the fringe portion can provide information about another aspect of the surface.
Beam-deflection is the most common detection method used in modern commercial probe microscopes because of its simplicity and versatility. Typically, beam deflection requires a force-sensing structure, a probe tip, a light source, and a photon detector. The force sensing structure can be a cantilever, a membrane, or any other substrate that measures displacement or force. Displacement of the force-sensing structure is translated into angular displacement by reflecting light from the backside of the force-sensing structure onto a photon detector. Such displacement can be related to the force imparted onto the sample or to the probe tip.
One modification to the beam-deflection method is to insert a one-dimensional diffraction grating into the path of the beam and measure angular displacement of diffraction spots rather than angular displacement of the reflected incident beam. Recent advancements in probe microscopy have combined a micro-machined membrane, transparent substrate, electrostatic actuator and a diffraction grating for optical interferometric detection. Currently a one-dimensional diffraction grating is used in conjunction with a membrane for such applications. During operation, only one diffraction spot is detected and related to the displacement or force of the membrane tip. An inherent assumption in the current design is that the membrane deforms uniformly in the radial direction when actuated or placed under load. Therefore the membrane measures only vertical displacements or forces.
Existing systems can provide information about the vertical topography of a surface and the vertical component of surface forces exerted by the object on the probe tip. However, many surfaces also impart lateral and even twisting forces on the probe tip. This may result in non-vertical displacement and deformation of the surface (i.e., the membrane or the cantilever) to which the probe tip is attached. Information about these lateral and twisting forces can provide valuable insight into the nature of a sampled surface. No existing system provides information about the degree or type of non-vertical displacement or deformation of the membrane or cantilever while it interacts with a surface.
Therefore, there is a need for a force microscopy system that provides information about non-vertical displacement and forces imparted on a probe tip by a sampled surface.
There is also a need for a microscopy system that provides information through both bright field analysis and dark field analysis of a diffracted beam.